This invention relates to articles having surface coatings with selectively controllable absorptance and emittance, and particularly such articles suitable for use in a space environment.
Spacecraft are exposed to a wide range of external thermal conditions, while functioning in various ways. Care must be taken to maintain the interior of the spacecraft within acceptable limits for the passengers and apparatus occupying the spacecraft. For example, one side of a satellite in earth orbit receives the direct, unfiltered rays of the sun during a portion of the orbital path, and the other side faces a range of conditions that can include the void of space. In another portion of the orbit when in the shadow of the earth, both sides may be exposed to cold conditions. Some parts of the interior of the spacecraft may be occupied by human beings, while other parts may be occupied by electrical apparatus producing large amounts of heat energy that requires a substantial amount of heat dissipation.
A number of active and passive techniques are used to moderate the effects of the radiation and temperature extremes on the interior of the spacecraft. There are active heating and cooling systems, but these can be heavy, bulky, and use a large amount of power. The walls are provided with passive insulation, but desirably the amount of insulation is relatively low because of its weight and bulk. Some spacecraft may be rotated to prevent excessive heat buildup, but in many other cases the spacecraft must maintain a specific orientation in order to perform its mission.
The heat energy reaching and leaving the spacecraft is transferred almost exclusively by radiation, inasmuch as there is no medium for heat transfer by conduction or convection. Therefore, in yet another passive thermal control approach, the exterior surfaces of the spacecraft may be provided with a coating, somewhat similar in character to a specialized paint, that alters the radiative transfer of heat to and from the exterior surfaces. Any coating applied to the exterior surfaces of the spacecraft must adhere well to the surfaces. It must also be stable in the space environment. That is, the components of the coating cannot change their essential character or be lost to the space environment during exposure to the temperature extremes and radiation of space. The coating must not rapidly deteriorate when exposed to the space environment, including UV (ultraviolet) radiation, particle radiation, atomic oxygen, micrometeorites, and orbital debris.
One prior thermal control coating provides a degree of variability in the absorptance thermal control property by including varying amounts of white and black pigments in a ceramic binder. Different compositions of the coating can be used on different spacecraft, and even on different regions of a single spacecraft, to achieve a degree of control over the radiative transfer to and from the surface of the spacecraft. While operable, such a coating falls short of providing the desired broad range of radiative properties required for some applications.
There is a need for an improved approach for passive thermal control of spacecraft. Such an approach should be capable of providing a wide range of thermal control properties and also satisfy the other requirements for use in a space environment.